1. Field of the Invention
The present invention relates to a liquid-crystal display device excellent in light utilizing efficiency, in brightness and in easiness of viewing, and to a light pipe which can be used for forming the liquid-crystal display device.
The present application is based on Japanese Patent Application No. Hei. 11-311916, which is incorporated herein by reference.
2. Description of the Related Art
Recently, discussion is made about a reflection-transmission double type liquid-crystal display device in which an illumination unit is provided so that the device can be viewed as a transmission type liquid-crystal display device in a dark while using the merit of a reflection type liquid-crystal display device which does not consume much electric power. For example, there is such a proposal that a liquid-crystal display device using a half-transmission type reflector, a liquid-crystal display device in which a back-lighting unit used in a transmission type liquid-crystal display device is disposed on the visual side of a liquid-crystal cell so as to be used as a front-lighting unit, and so on.
The system using a half-transmission type reflector, however, had a disadvantage in that the system was inferior in brightness in either mode to an exclusive reflection or transmission type liquid-crystal display device because light was separated into reflected light and transmitted light by the half-mirror effect of the half-transmission type reflector. There might be a proposal to perform improvement by use of a reflection polarizer which reflects polarized light so selectively that the sum of reflectance and transmittance can exceed 100%. Light utilizing efficiency in a transmission mode was, however, lowered to 50% or less because of absorption by a light absorber disposed for preventing reversal of display between reflection and transmission modes and for preventing drifting of black display. There was a problem that display was hardly viewed in either mode at twilight. Also in the front-lighting system, display was apt to be dark in a transmission mode compared with the general transmission type liquid-crystal display device because light went to the liquid-crystal cell, or the like, and came back. There was a problem that a fault or dirt on a light pipe became conspicuous as a bright spot. Moreover, there was a problem that display contrast was lowered by light leaking from the upper surface of the light pipe.
An object of the present invention is to develop a liquid-crystal display device of good visibility excellent in brightness in both reflection and transmission modes and free from generation of reversal of display and lowering of contrast due to leaking light.
According to the present invention, there is provided a liquid-crystal display device comprising: a light pipe including an upper surface, a lower surface opposite to the upper surface, an incidence side surface and a light output means formed in the upper surface, the light pipe having a retardation of not larger than 40 nm between the upper and lower surfaces; a light source disposed on the incidence side surface; a reflection layer disposed on the lower surface; and a liquid-crystal shutter disposed on the upper surface and including a liquid-crystal cell and at least one polarizer, whereby light incident on the incidence side surface from the light source goes out from the lower surface through the light output means, the outgoing light is then reflected on the reflection layer, and the reflected light is then transmitted through the light pipe so as to go out from the upper surface finally.
According to the present invention, there is provided a structure in which a light pipe free from generation of the retardation is disposed between a liquid-crystal cell and a reflection layer. Hence, the state of linear polarization through a polarizer hardly changes, so that display irregularity due to variation in brightness, irregularity in color, etc. hardly occurs. Moreover, lowering of light utilizing efficiency in a reflection mode is so slight as represented by absorption loss, reflection loss, etc. due to the light pipe. Hence, brightness substantially equal to that of the background-art reflection type liquid-crystal display device can be achieved in the reflection mode. Moreover, brightness equal to that of the background-art transmission type liquid-crystal display device can be achieved also in a transmission mode.
A liquid-crystal display device of good visibility can be obtained without occurrence of reversal of display due to reflection and transmission and without occurrence of lowering of contrast due to light leaking from the light pipe. Moreover, the provision of a light output means in the upper surface of the light pipe permits the optical path in the light pipe to be elongated in a transmission mode. Hence, the spread of light can be widened, so that the intensity of the emission lines can be relaxed. This acts advantageously on prevention of moire and improvement of uniformity of display. The reflection layer can be disposed closely and easily on the lower surface of the light pipe through an adhesive layer, or the like, so as to be integrated with the light pipe.
In addition, in the case of a light pipe having a light output means constituted by slopes such as prism-like irregularities, the light pipe is excellent in directivity of light reflected by the slopes. Hence, light favorable to viewing in a transmission mode can be generated efficiently, so that brighter display can be obtained. Moreover, the light pipe is excellent in incidence efficiency of external light and transmittance after reflection. Hence, brighter display can be obtained also in a reflection mode by light emission good in light utilizing efficiency and excellent in uniformity. Moreover, the occurrence of moire due to the directivity can be suppressed by oblique arrangement of the light output means. Hence, glaring light can be prevented from disturbing visual recognition.
If the retardation due to the light pipe is large in the above description, linearly polarized light through a polarizer of a liquid-crystal shutter is elliptically polarized in a reflection mode because of the retardation while the linearly polarized light goes to the light pipe and comes back through the reflection layer. Hence, an absorbed component is created when the light enters the polarizer again. As a result, brightness is lowered. Moreover, coloring is generated by wavelength dispersion due to the retardation. Moreover, if the retardation varies in accordance with the position and direction of transmission of light because of variation in the retardation, brightness varies in accordance with the position and direction of transmission of light. This causes display irregularity. Moreover, variation in color is created because of colorlessness and coloring to different colors such as navy blue, blue, etc. in accordance with the position and direction of transmission of light due to the wavelength dispersion. This also causes display irregularity.
On the other hand, in the case where a light output means is provided in the lower surface of the light pipe, from the point of view of keeping the function of the light output means it is necessary that an independent reflector is disposed separately. The light pipe becomes complex in structure because of the arrangement and fixation of the reflector as well as the number of parts is increased. Hence, a thick support is required for preventing display from being disordered by wrinkling. There is a disadvantage in that the light pipe becomes heavy.
Further, in the case of a light pipe having a scattering type light output means such as dot-like or embossed irregularities, display is hard to view in a frontal (perpendicular) direction in a transmission mode because light outputs at a large angle of about 60 degrees. If a prism sheet is disposed for the purpose of controlling the optical path, display becomes very dark in a reflection mode because light is scattered so that a greater part of light does not contribute to the display. Moreover, dots, or the like, are viewed clearly. If a diffusing layer having strong diffusing characteristic is disposed for preventing the dots from being viewed clearly, display becomes dark because both the incident light in the reflection mode and the light reflected by the reflection layer are scattered.